1. Field of the Invention
The present invention relates to a contact lens for correcting astigmatism or presbyopia. More particularly, it relates to a toric contact lens or a multifocal (bifocal) contact lens capable of preventing the rotation of it on the eye during use.
The astigmatism is a state that parallel beams do not form an image at a point and the presbyopia is a state that a man becomes unable to attain a sharp focus for near vision due to the reduction of elasticity of the crystalline lens. In order to correct the astigmatism or presbyopia, glasses or a contact lens is usually used.
2. Discussion of the Background
As a contact lens to correct the astigmatism, there is known a toric contact lens wherein the radius of curvature of the inner (back) and/or the outer (front) surfaces is different from each other in terms of two orthogonally intersecting meridians.
As a contact lens to correct the presbyopia, there is known a multifocal (bifocal) contact lens wherein a plurality of areas of the degree of lens power are formed at the inner (back) and/or outer (front) surfaces in order to see near and distant objects.
For a patient of astigmatism, it is necessary to wear a toric contact lens at a correct position on a human eye.
There are various kinds of multifocal contact lens to be worn by patients of presbyopia, some kinds of multifocal contact lens have to be worn at a fixed position to the human eyes in the same manner as the toric contact lens. The multifocal contact lens of the above-mentioned kind is such that the areas of the different degrees of lens power formed in the contact lens are not at the positions of rotation symmetry when the lens is viewed from the front side. When glasses are used, a frame holding lenses is fitted to the ears or the nose. Accordingly, the correct position of the lenses can be assured. However, when a toric contact lens or a certain kind of multifocal contact lens is used, there is no means to fix the lenses, and accordingly, the contact lenses tend to rotate on the eye, whereby it may not obtain proper correction.
For the toric contact lens or the multifocal contact lens, there have been proposed various improvements to prevent the rotation of the lens on the eye. The proposed improvements can be generally classified into two types.
The first type is called a ballast type wherein the lower portion of the contact lens is relatively made heavy to thereby prevent the rotation of the lens on the eye (the cornea). A bifocal (multifocal) type contact lens 31 (FIG. 7) which belongs this type, has been known (for instance, in Japanese Examined Patent Publication No. 18531/1962).
The second type is such one as to correspond to the shape of the eye of a wearer so that the rotation of the lens on the cornea can be prevented. The second type can also be classified into two types: a truncation type and a slab-off type. In the truncation type, a cut portion (a truncated portion) 33 is formed at a part of the outer periphery of a contact lens (FIG. 8), which is disclosed in, for instance, Japanese Unexamined Utility Model Publication No. 13048/1973). In the slab-off type, a thin portion 35 is formed in a contact lens 34 (FIG. 9) which is disclosed in, for instance, Japanese Unexamined Utility Model Publication No. 133151/1975.
In the ballast type contact lens, the thickness in a portion of the lens is increased to shift the gravity center to the lower portion. Therefore, the ballast type contact lens has a disadvantage that a wearer may feel a pressure on the eye, discomfort or pain. In order to improve the discomfort, a so-called prism ballast type contact lens is proposed wherein the central axis of the outer surface of the contact lens is deviated from the central axis of the inner surface so that the thickness of the lens is gradually increased downwardly. The proposed prism ballast type contact lens has, however, such a disadvantage that when a lens is designed in accordance with the parameters (such as the degree of lens power), there may result a thicker portion in the lens, to which a wearer cannot endure. Further, as the thickness of the lens is increased, an amount of oxygen to be transmitted therethrough is reduced, whereby the cornea may be injured.
On the other hand, since the truncation type contact lens is so formed that the peripheral region of the lens in a plane view is not smooth, a wearer may feel discomfort when the lens is worn. In order to improve the feeling of wearing, it is necessary to finish the truncated portion smoothly. However, it requires high skill and technique. Further, even though a truncated portion having a smooth surface is formed, sufficient effect cannot be expected depending on the shape of eyelid of a wearer.
In the slab-off type contact lens, there is no problem on the feeling of wearing. However, the contact lens has its gravity center which coincides with geometric center. Accordingly, there is a problem of stability because the lens may rotate on the cornea when the eye blinks.
In order to solve the problems of the ballast type, the truncation type and the slab-off type contact lens as described above, a technique in combination of the ballast type and the slab-off type has been proposed as follows.
Firstly, in U.S. Pat. No. 4,324,461, a thin wall portion (hereinbelow, referred to as a slab-off portion) is formed at the upper portion of a lens to shift the gravity center downwardly.
Secondly, in U.S. Pat. No. 5,020,898, slab-off portions are formed at the upper and lower portions of a lens to thereby shift the gravity center downwardly. In these proposals, however, it is difficult to assure a proper deviation of the gravity center necessary for preventing the rotation of the lens on the cornea. Further, excellent and stable eyesight may not be obtained.
There is another proposal wherein a slab-off portion is formed in the lower portion of the lens and a prism is formed in the lens to make the lower portion of the lens heavy (U.S. Pat. Nos. 4,508,436 and 4,573,774). In the proposal, however, the position of the gravity center is moved upwardly since the slab-off portion is formed in the lower portion of the lens, whereby there is little effect to prevent the rotation of the lens on the eye.